A hierarchical storage management (HSM) for efficiently utilizing the storage capacity of a file system (Patent Document 1) has been well known. The HSM means control for storing file data by using a combination of a plurality of storage devices with different performances, capacities and/or reliability. Focusing on the performances and capacities, the HSM uses at least two types of storage devices: a high-speed storage device with better performance than that of a low-speed storage device described hereinafter, and a low-speed storage device with a lower unit price per capacity than that of the high-speed storage device mentioned above. Note that the unit price per capacity is value obtained by dividing the storage capacity of each storage device by the price of the storage device. Hereinafter, the storage capacity is often referred to simply as “capacity.” Examples of the performances include response time and transfer speed. The plurality of storage devices form groups based on the performances and the unit price per capacity, and each of the groups is called “tier.” For example, a group that is configured by the storage devices with performances higher than a certain standard is often called “higher level tier,” and a group that is configured by the storage devices with unit price per capacity lower than a certain standard is often called “lower level tier.”
In the HSM, file data received from a client computer is first stored in the higher level tier. Thereafter, the shortage of the capacity of the higher level tier with high performance and unit price per capacity is alleviated by specifying a file, the last access time (e.g., creation time, update time, reference time) of which is earlier than the current time by at least a predetermined time period, and moving the specified file to the lower level tier. Note that this movement of a file from the higher level tier to the lower level tier is often called “tier descent movement,” and the predetermined time period mentioned above is often called “tier descent base time period.” Generally, when the file data is referenced or updated, the usage frequency thereof tends to decline as the time elapses since the creation time of the file data. For this reason, when moving a file according to the access time as described above, a file with a low reference frequency is expected to be moved from the higher level tier to the lower level tier.
Note that “time” described in the present specification may include information such as the year, month and date, or may be expressed in a form other than “year, month, date, hour, minute, second,” as long as the time elapsed since a predetermined base can be expressed. For example, on the basis of “0 hour 0 minute 0 second on January 1, year 0,” the time may be expressed in seconds, or in counters, such as several microseconds, several milliseconds, and seconds as in a general computer, on the basis of “0 hour 0 minute 0 second on Jan. 1, 1970.”
Note that examples of the types of access to the files in the present specification include file generation, data addition, data update, data reference, and file deletion, and may also include other file operations as the types of access. Note that the types of access may not necessarily include all of the above examples, because some models of file storage systems purposefully prohibit or do not implement the operations such as file deletion and file update.
A stub file is used in order to easily refer to the file data moved from the higher level tier to the lower level tier (Patent Document 2). The stub file is disposed in the higher level tier. The stub file is the information for indicating a place in the lower level tier for storing the file data moved to the lower level tier. A user can refer to the file data moved to the lower level tier, by referring to the stub file stored in the higher level tier. Due to this stub file, a path name designated by the client computer when the stub file is stored in the higher level tier remains unchanged even after the file data is moved to the lower level tier. The path name is not changed also before and after performing a recall process on a file that is subjected to the recall process.
There is also known a technology for using metadata of a file to generate a virtual view for providing the user with files and directories using a variety of views (Patent Document 3).